In a variety of manufacturing applications, it is necessary to grip and hold a workpiece to enable an operation to be performed on the workpiece. For example, in a stretch forming operation, a workpiece is typically gripped at opposite ends in a pair of gripping devices and the gripping devices are moved apart so as to place the workpiece in tension. The tensioned workpiece is then bent around a forming member to form a desired shape.
One approach that has been used for gripping and holding a workpiece for stretch forming is to fabricate a set of complementary jaws for each configuration of workpiece that is to be gripped. The set of jaws matching a given workpiece shape is placed into a jaw holding device, generally known as a jaw head, so that the workpiece can be gripped between the jaws. In this type of gripping device and method, the gripper jaws must be precisely manufactured to the exact shape complementing that of the workpiece in order to enable the jaw head and gripper jaw to properly function. As a consequence, each configuration of workpiece to be gripped requires its own set of dedicated gripper jaws. If a substantial number of different workpiece configurations are to be worked on, it can become quite expensive to fabricate and maintain the required inventory of different gripper jaw sets. Another drawback to this approach is that variation in tolerances of the gripper jaws can cause unacceptable variances in fit and function of the jaws. Furthermore, removing one set of gripper jaws and installing a new set of jaws configured for a new workpiece configuration result in considerable amounts of wasted time.
Another approach that is used for gripping workpieces is to install in the jaw head a gripper jaw that has a shape roughly complementary to that of the workpiece. The workpiece is then modified in shape in the area to be gripped to match it to the shape of the gripper jaw. This approach, however, adds considerable time to the manufacturing process for the workpiece, and also requires undesirable modifications to the workpiece in many cases. In some instances, the requisite modifications to the workpiece to fit the gripper jaw may result in the modifications being transferred to other areas of the workpiece during manufacturing operations, which is generally undesirable.
A further drawback common to each of the above approaches is that the gripper jaws are manufactured to accommodate a particular thickness of workpiece. Accordingly, where it is desired to grip workpieces of various thickness, a number of different gripper jaws must be provided, each tailored to the thickness and shape of one particular workpiece configuration.